The present invention relates to a corrosion-inhibiting coating, process for creating the corrosion-inhibiting coating, and a corrosion-inhibiting coating bath. More specifically, the present invention relates to a coating, process, and system using zinc- or a zinc-alloy as an adherent that is directly deposited onto a steel surface for enhanced corrosion protection.
Steel or cast iron materials such as those used for fasteners, automotive bodies, and industrial processing equipment require protection from corrosion due to the low oxidation-reduction (redox) potential of iron. Typically, these materials are coated with a thin “sacrificial” coating of a material with an even lower redox potential. The two materials that are typically used as sacrificial materials for steels are cadmium and zinc, or alloys composed of the same. During corrosive attack, these cadmium or zinc sacrificial materials are themselves preferentially corroded, maintaining the structural integrity of the underlying steel.
In instances where these sacrificial materials are removed from the steel surface, corrosive attack of the underlying steel will begin. For example, if the zinc layer which protects the steel is removed, then the underlying steel begins to corrode. Additionally, if the steel is galvanically coupled to a third metal that has a lower redox potential than iron, then that third metal will begin to corrode once the “sacrificial” layer of zinc or cadmium is removed. This process is frequently observed during aircraft maintenance procedures. Cadmium-plated steel fasteners are used in aluminum alloy wing and fuselage sections. During routine maintenance, the cadmium plate is frequently removed from the fasteners, setting up a steel-aluminum galvanic couple. This inevitably results in corrosion of the lower redox potential material (aluminum).
A method of replacing this sacrificial layer over the steel surfaces is therefore advantageous for many applications. Zinc and zinc-containing alloys are preferred for this application, due to the toxic nature of cadmium. However, the conventional methods of applying zinc (e.g., electroplating or hot-dip processes) are not suitable for this application because neither is practical to repair the steel piece without removal of that part or steel piece. In addition, both processes require a large degree of energy expenditure in order to perform a simple repair operation. Therefore, replating an automotive or aircraft component in the field in order to replace this sacrificial layer will require a large electrical expenditure. The application of a molten zinc layer to a structure in need of repair requires a high temperature (419.5° C.), but this high temperature may damage other structural components.
Another method involves the incorporation of zinc dust or a zinc metal-zinc oxide mixture within a polymer film (e.g., a paint), which is then applied directly onto the steel surface (i.e., zincrometal). This severely limits the successful application of conversion or phosphate coatings for subsequent paint application. In order to function properly, conversion or phosphate coatings must be applied directly onto a metal surface (e.g., zinc). Application of barrier films that contain zinc dust may result in superior corrosion protection, but the resultant adhesion to such barrier film coatings is poor.
Kimura et al. (U.S. Pat. No. 5,116,664) teaches using electroless plating where the electroless plating bath contains a metal salt, including zinc salts, and a reducing agent, including sodium hypophosphate. The electroless plating bath may also contain chelating stabilizers and buffers. However, Kimura teaches using such plating system to create a titanium-mica composite material and not for corrosion protection of steel surplus. Also, Kimura does not disclose using a fluoride preparative in his bath.
Thus, there is a need in the art for a coating which provides superior adhesion and corrosion protection for steel surfaces.